The long-term goal of this research is to elucidate the detailed mechanistic basis for ligand binding and release by the low-density lipoprotein receptor (LDLR), and to understand signal transduction by structurally related proteins of the LDLR family. The LDLR is the primary mechanism for uptake of cholesterol carrying particles into cells; the genetic disease familial hypercholesterolemia (FH), which affects about 1 in 500 persons worldwide, results from loss-of-function mutations in the LDLR gene. The LDLR also serves as a prototype for a versatile family of cell-surface receptors that participate in a diverse range of biological processes, including lipoprotein uptake, brain development, and Wnt signal transduction. These proteins combine several types of structural units in similar arrangements, such that groups of cysteine rich LDL-A modules precede regions with clusters of epidermal growth factor-like (EGF) modules and B-propeller domains containing conserved YWTD motifs. Each receptor then terminates with a transmembrane segment and a cytoplasmic tail of variable length. Although these receptors carry out a variety of crucial biological functions, the basis for ligand recognition by the ligand binding, LDL-A modules of the LDLR and related receptors remains poorly understood. In addition, the mechanism by which the LDLR releases ligands in the acidic milieu of the endosome, a process intrinsic to the purified receptor and mediated by the EGF-like modules and the YWTD domain, is also unknown. During the next period of grant support we will address these unanswered questions in lipoprotein receptor biochemistry by pursuing the following specific aims: 1. Determine how the ligand-binding modules of the LDL receptor recognize apolipoprotein E (apoE)-containing ligands. 2. Elucidate the mechanism of ligand release by the LDLR at low pH.